The present disclosure relates to extracorporeal blood circuits, systems, and methods of use. More particularly, it relates to devices for oxygenating and filtering blood in an extracorporeal blood circuit, and methods of making such devices.
An extracorporeal blood circuit is commonly used during cardiopulmonary bypass to withdraw blood from the venous portion of the patient's circulation system (via a venous cannula) and return the blood to the arterial portion (via an arterial cannula). The extracorporeal blood circuit generally includes a venous drainage or return line, a venous blood reservoir, a blood pump, an oxygenator, an arterial filter, and blood transporting tubing, ports, and connection pieces interconnecting the components. As shown in FIG. 1, some prior art extracorporeal blood circuits drain venous blood from patient 10 via a venous return line 12. Cardiotomy blood and surgical field debris are aspirated from the patient 10 by a suction device 16 that is pumped by a cardiotomy pump 18 into a cardiotomy reservoir 20. Venous blood from the venous return line 12, as well as de-foamed and filtered cardiotomy blood from the cardiotomy reservoir 20, are discharged into a venous blood reservoir 22. Air entrapped in the venous blood rises to the surface of the blood in the venous blood reservoir 22 and is vented to atmosphere through a purge line 24. A venous blood pump 26 draws blood from the venous blood reservoir 22 and pumps it through an oxygenator 28 and an arterial blood filter 29. An arterial line 14 returns the oxygenated and filtered blood back to the patient's arterial system via an arterial cannula (not shown) coupled to the arterial line 14.
The oxygenator component of the extracorporeal blood circuit is well known. In general terms, the oxygenator takes over, either partially or completely, the normal gas exchange function of the patient's lungs. In oxygenators that employ a microporous membrane, blood is taken from the patient and is circulated through the oxygenator on one side of the membrane. Concurrently, an oxygenating gas is passed through the oxygenator on the other side of the membrane. Carbon dioxide diffuses from the blood across the microporous membrane into the passing stream of oxygenating gas; at the same time, oxygen diffuses from the oxygenating gas across the membrane into the blood. The circulating blood, having thereby been reduced in carbon dioxide content and enriched in oxygen, is returned to the patient. One popular type of membrane oxygenator is referred to as a hollow fiber oxygenator, and is illustrated generally in U.S. Pat. No. 4,239,729. A hollow fiber oxygenator employs a large plurality (typically tens of thousands) of microporous or semipermeable hollow fibers disposed within a housing. These hollow fibers are sealed in end walls of the housing that are then fitted with skirted end caps. One end cap is fitted with an inlet, the other end cap is fitted with an outlet. A peripheral wall of the housing has an inlet located interiorly of one of the end walls and an outlet located interiorly of the other end wall. The oxygenating gas enters the device through the inlet, passes through the lumens of the hollow fibers, and exits the device through the outlet. It will be understood that carbon dioxide diffuses from the blood flowing over the outer surfaces of the hollow fibers through the fiber walls and into the stream of oxygenating gas. At the same time, oxygen from the oxygenating gas flowing through the lumens of the hollow fibers diffuses through the fiber walls and into the blood flowing about the fibers to oxygenate the blood.
A well-accepted technique for forming a hollow fiber oxygenator is to spirally wind ribbons of the fibers about an internal supporting core, as described for example in U.S. Pat. No. 4,975,247. Blood flow through the resultant annular “bundle” of fibers can be in various directions such as radially outward, axial, circumferential, etc. With radially outward flow designs, U.S. Pat. No. 5,462,619 describes an improved winding technique that provides desired pressure drops and minimal clotting risks by a graduated packing fraction. An oxygenator product available from Medtronic, Inc., under the trade name Affinity® NT Oxygenator, is one example of a spirally wound hollow fiber oxygenator with graduated packing fraction.
For purposes of this disclosure, packing fraction is defined to mean the fraction of a unit volume of bundle space occupied by fibers (or filaments). The packing fraction may be determined in ways known in the art, including the convenient method of measuring the interstitial space between fibers (or filaments) by weight gain when a unit volume is primed with a known liquid. Packing fraction at a particular region or zone located radially outward may be determined by stopping the corresponding winding process at the radially inner radial boundary of the region or zone and determining the packing fraction at that stage, and then continuing the winding process to the outer radial boundary of the region or zone and determining the packing fraction at that stage. Computations known in the art will determine the packing fraction of the region or zone using the prior two values.
Arterial filters are also well known, and can take various forms appropriate for air handling and blood filtration. In general terms, the conventional arterial filter device includes one or more screen-type filters within a filter housing that combine to capture and remove particulate (e.g., emboli) on the order of about 20-40 microns and larger, as well as to trap gaseous microemboli larger than a certain size to prevent the emboli from reaching the patient. These emboli can cause significant harm to the patient by plugging small arteries, arterioles, and or capillaries, preventing adequate blood flow to small or large areas of tissue or organs. Examples of known arterial blood filters are described in U.S. Pat. Nos. 5,651,765 and 5,782,791. Arterial blood filters are also available from Medtronic, Inc. under the trade name Affinity® Arterial Filter.
Conventionally, the arterial filter device is fluidly connected within the extracorporeal circuit downstream (or upstream) of the oxygenator device by tubing. While implementation of the separate oxygenator and arterial filter devices as part of an extracorporeal blood circuit is well accepted, certain concerns arise. An arterial filter typically adds 200 ml (or more) of prime volume to the extracorporeal blood circuit; this added prime volume is undesirable as it can lead to increased hemodilution of the patient. As a point of reference, the volume of blood and/or prime solution liquid that is pumped into the extracorporeal blood circuit to “prime” the circuit is referred to as the “prime volume”. Typically, the extracorporeal blood circuit is first flushed with CO2 prior to priming. The priming flushes out any extraneous CO2 gas from the extracorporeal blood circuit prior to the introduction of the blood. The larger the prime volume, the greater the amount of prime solution present in the extracorporeal blood circuit that mixes with the patient's blood. The mixing of the blood and prime solution causes hemodilution that is disadvantageous and undesirable because the relative concentration of red blood cells must be maintained during the surgical procedure in order to minimize adverse effects to the patient. It is therefore desirable to minimize the extracorporeal blood circuit's prime volume (and thus the required volume of prime solution).
In light of the above, a need exists for an extracorporeal blood circuit device that provides oxygenation and arterial filtering properties at least commensurate with conventional oxygenator and arterial filter components, yet minimizes the overall impact on the prime volume of the extracorporeal blood circuit.